This invention relates generally to methods and apparatus for reconstructing image data, and more particularly to methods and apparatus for image reconstruction in a computerized tomography (CT) imaging system providing reduced X-ray exposure as compared to conventional CT imaging systems.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Exposure to x-rays in CT imaging systems may present a hazard to a patient. At least in the long term, it may also present a hazard to a physician performing a procedure in the vicinity of a CT imaging system. Current CT systems provide tomographic cross-sections of a patient with a field of view that is normally around 50 cm, and a gantry opening of 70 cm. For applications in imaging a small organ such as a heart, exposing a patient with X-ray photons across the whole cross-section of the patient where the region of interest is a small organ may not be justifiable.
Several techniques have been proposed to reduce the total exposure risk. For example, the x-ray source may be turned on only when the source is on its lower trajectory, where both primary and scatter are more likely to be attenuated by the patient table. Often, after initial localization and insertion of a biopsy needle, a physician is interested in a specific and targeted anatomy region. Although turning on the x-ray source on its lower trajectory limits radiation exposure of both the patient and the physician, it still exposes more of the patient to x-rays than is desirable and does not fully shield the physician from exposure. Exposure to x-ray radiation could be reduced through the use of an x-ray source limited in fan-angle coverage to the region of interest (ROI) of the patient. The data resulting from the limited X-ray source would then be limited in terms of fan-angle coverage. However, no previously known method or apparatus provides reconstruction, from such limited data, with the image quality typical of a state-of-the art CT scanner. When direct reconstruction is attempted from such limited data, a very large object-dependent shading is introduced over the ROI, rendering the image data useless.
It would therefore be desirable to provide a reconstruction method and apparatus that provided image reconstruction from limited projection data obtained from a CT scanner. In particular, it would be desirable to obtain such high-quality reconstruction of a region of interest from data obtained from a beam of limited fan-angle extent, or from limited exposure to a wider, collimated beam. In addition, because the ROI may not be directly centered in a beam from an X-ray source of a CT scanner, it would be desirable to provide a method and apparatus for transposing the ROI into the center of the beam without shifting the patient relative to the scanner bed.